1. Field of the Invention
The invention relates to optical receivers, and in particular to RF amplifiers that have a non-linear operating characteristic and interface between a photodiode that receives an optical signal from an optical fiber, such as used in fiber optical communications links, and electronic network terminal units.
2. Description of the Related Art
A variety of optical communications systems are known in the art which include an optical transmit portion that converts an electrical signal into an optical signal launched into an optical fiber, and a receive portion that receives an optical signal from the optical fiber and converts it into an electrical signal. Traditionally, optical receive sections include an optical assembly to focus or direct the light from the optical fiber onto a photodetector, which in turn, is connected to an amplifier/limiter circuit on a circuit board. The photodetector or photodiode is typically packaged in a hermetically sealed package in order to protect it from harsh environmental conditions. The photodiodes are semiconductor chips that are typically a few hundred microns to a couple of millimeters wide and 100-500 microns thick. The package in which they are mounted is typically from three to six millimeters in diameter, and two to five millimeters tall and has several electrical leads coming out of the package. These electrical leads are then soldered to the circuit board containing the amplifier/limiter and other circuits for processing the electrical signal.
Fiber optic transmission has been used for many years for the distribution of analog CATV signals. Optical receivers for analog CATV signals typically consist of a packaged photodiode electrically connected to a separately-packaged low-noise electronic amplifier, with the photodiode package usually consisting of a TO can with an optical window cap of a ball lens cap to allow light from an optical fiber to be coupled to the photodiode.
A disadvantage of the above arrangement is that there are significant parasitic capacitances associated with the separate photodiode and electronic amplifier packages. These parasitic capacitances play a role in limiting the performance of the optical receiver, resulting in lower bandwidth and higher noise, and contributing to higher power dissipation required to meet a particular target specification.
Distribution of analog CATV signals is particularly challenging because of the high degree of linearity required to obtain adequate quality. Two techniques have been employed previously to obtain this level of low electrical distortion.
Operating the amplifying transistors with high quiescent currents and voltages so that the signal swing is limited to a small fraction of the available current or voltage excursion. These high quiescent voltages and currents lead to high DC power dissipation.
Use of a push-pull or balanced amplifiers, which cancel even-order distortion products are also known. Push-pull amplifiers also come at the price of higher power dissipation, however, because two amplifiers are needed instead of just one single-ended amplifier, and also at the price of balanced-to-unbalanced transformers needed to interconnect to later single-ended stages.
Another technique that has been used in CATV systems is pre-distortion. Here an electrical signal is generated prior to the nonlinear element such that it cancels the distortion of the nonlinear element. This has been mainly applied to linearize lasers.
For new Fiber-to-the-Home applications that include an analog optical CATV receiver at each customer premise, there is a strong desire on the part of system suppliers and operating companies to reduce power dissipation and to lower device cost.